Preparation of rubber-modified molding materials by means of groups which are incorporated in the rubber and form free radicals on thermal decomposition

ABSTRACT

In the preparation of a rubber-modified molding material, in a first stage, a first mixture (A) contains at least one alkyl acrylate or methacrylate (a) of the formula (I) ##STR1## where R 1  is hydrogen or methyl and R 2  is alkyl of 1 to 32 carbon atoms, at least one first monomer (b) which forms free radicals on thermal decomposition and, if required, a second monomer (c) or a plurality thereof is polymerized, preferably with free radicals, to give a rubber (B), 
     in a second stage, the resulting rubber (B) is mixed with a third monomer (d) or a plurality thereof, preferably dissolved or swollen therein, to form a second mixture (C), and 
     in a third stage, the second mixture (C) is polymerized with free radicals to give the rubber-modified molding material (D).

The present invention relates to a process for the preparation ofrubber-modified molding materials, groups which form free radicals onthermal decomposition being incorporated in the rubber, and torubber-modified molding materials prepared by the process.

Rubber-modified molding materials are materials in which domains ofelastomers, for example rubbers, are embedded in a matrix comprising athermoplastic. There is considerable demand for rubber-modified moldingmaterials which have surface gloss, impact strength and tensilestrength. The characteristic domain structure is responsible for thedesired mechanical properties.

The multiphase character and hence also the domain structure ofrubber-modified molding materials is based on the fact that they arecomposed of different polymer components which are immiscible or onlypartly miscible with one another. The impact strength thereof resultsfrom increased energy absorption during deformation up to fracture. Theenergy is consumed for the formation of microcavities or for initiatingsliding processes of the matrix polymer chains. The multiphase characteris therefore essential for achieving high impact strengths.

The following are also applicable:

1. The two chemically different polymer components form a stabledispersion of defined particle size, which neither exhibits phaseseparation in the thermoplastic melt (processing) nor tends tohomogenization with formation of a macromolecular solution at hightemperatures.

2. Coupling must occur between the elastomer particles and the matrix,i.e. it must be possible to transmit forces at the phase boundaries.

The most effective phase coupling at the boundaries of the elastomerparticles is achieved by graft copolymerization. For example, in thepreparation of acrylonitrile/styrene/acrylate (ASA) molding materials,the procedure generally adopted is one in which an acrylate rubber isinitially taken and, by polymerization with a monomer mixture comprisingstyrol and acrylonitrile, copolymers based on the latter two monomersare then grafted on.

EP 0 095 919, EP 0 143 991, EP 0 438 418, EP 0 001 782, DE 11 828 11,JP-A 71003-182, JA 60 21 0666 and B. Vollmert, AngewandteMakromolekulare Chemie, 3, 18 (1968), 1-27, describe the preparation ofthermoplastics modified with acrylate rubbers.

It is an object of the present invention to provide a process for thepreparation of rubber-modified molding materials having improvedmechanical properties, in particular higher impact strengths, notchedimpact strengths and hole impact strengths.

We have found that this object is achieved by the novel process inwhich, in a first stage, a first mixture (A) which contains at least onealkyl acrylate or methacrylate (a) of the general formula (1) ##STR2##where R¹ is hydrogen or methyl and R² is alkyl of 1 to 32 carbon atoms,at least one first monomer (b) which forms free radicals on thermaldecomposition and, if required, a second monomer (c) or a pluralitythereof is polymerized, preferably in solution, with free radicals togive a rubber (B),

in a second stage, the resulting rubber (B) is mixed with a thirdmonomer (d) or a plurality thereof, preferably dissolved or swollentherein, to form a second mixture (C), and

in a third stage, the second mixture (C) is polymerized with freeradicals to give the rubber-modified molding material (D). Thepolymerization being carried out either continuously as a solution ormass polymerization or being continued as a suspension polymerizationafter a conversion of at least 15%, the rubber-modified molding material(D), containing from 1 to 60% by weight of the rubber (B) and therubbers (B) in grafted form being present in said molding material (D)as particles having a diameter of from 0.1 to 20 μm.

The first monomer (b) which forms free radicals on thermal decompositionshould form only a few free radicals, if any at all, during its ownpolymerization.

The novel process has the advantage that, apart from the abovementionedproperties, the weather resistance and the rigidity of the moldingmaterials is also improved. By varying the preparation parameters, thegloss of the molding materials prepared can be varied over a wide range.

A graft copolymerization takes place in the third stage, the backbone ofthe resulting graft copolymer being formed by the rubber (B), and thegrafts being composed of the third monomer (d). Because 100% graftingdoes not occur, some of the rubber remains ungrafted and at the sametime an ungrafted chain polymer is formed from the third monomer ormonomers (d) and constitutes the hard matrix. There are therefore atleast three types of molecules in the molding material.

The groups incorporated in the rubber via the first monomer (b) formfree radicals on thermal decomposition and therefore act as free radicalinitiator in the third stage, so that it is now assumed that a largernumber of grafts is formed compared with rubbers without these groups.In the case of the groups which are incorporated according to theinvention and form free radicals, the temperature at which they have ahalf-life of one hour is so high that they are stable in the processbefore the third stage, in particular at from 80 to 200° C., preferablyfrom 80 to 150° C. Examples of such groups are peroxy groups, diazogroups and labile C--C bonds. Examples of suitable monomers (c) whichcontain these groups are tert-butyl 3-isopropenylcumyl peroxide,tert-butyl peroxycrotonate and tert-butyl monoperoxymaleate, the firsttwo being preferred.

The determination of the temperature at which the monomers (b) have ahalf-life of one hour is described in the brochure entitled Initiatorsfor Polymer Production from AKZO (Code: 92.10.10891).

Examples of alkyl groups R² of 1 to 32 carbon atoms are methyl, ethyl,n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl,n-hexyl, n-heptyl, n-octyl, ethylhexyl, n-nonyl, n-decyl, n-dodecyl,n-tetradecyl, n-hexadecyl, n-octadecyl, n-eicosyl, n-docosyl,n-tetracosyl, n-hexacosyl, n-octacosyl, n-triacontyl, n-hentriacontyland dotriacontyl.

In a preferred embodiment of the invention, R² is alkyl of 1 to 18carbon atoms.

In a further preferred embodiment of the invention, the mixture (A) hasthe following composition:

from 30 to 99.95% by weight of the acrylate or methacrylate (a),

from 0.05 to 10% by weight of the first monomer (b) and

from 0 to 60% by weight of the second monomer (c);

the preferred ranges are:

from 36 to 99.5% by weight of (a),

from 0.1 to 4% by weight of (b) and

from 0 to 60% by weight of (c).

The rubber (B) preferably has a glass transition temperature of lessthan 0° C., preferably less than -10° C., the glass transitiontemperature being determined by means of DSC according to ASTM 3418. Therubber thus has the required softness. The glass transition temperaturecan be established either by using an acrylate or methacrylate whosepolymer has the desired glass transition temperature or by using amixture of acrylates or methacrylates which have different lengths ofthe side chains (R² in the formula I). This adjustment of the glasstransition temperature is based on the fact that the glass transitiontemperature of acrylate and methacrylate polymers initially decreaseswith increasing length of the side chain, then passes through a minimumand finally increases again. The minimum occurs at a side chain of about7 carbon atoms for polyacrylates and at a side chain of 10 carbon atomsfor polymethacrylates. This general range for the length of the sidechain R² is therefore preferred. The rubber-modified molding material(D) prepared contains from 1 to 60, preferably from 5 to 40, % by weightof the rubber (B). The upper limit is determined by the fact that themolding material must have sufficient strength in spite of the embeddeddomains of the rubber. The lower limit is determined essentially by thefact that sufficient energy must be absorbed during deformation.

In a further preferred embodiment of the invention, the alkyl acrylateor methacrylate (a) used is n-butyl acrylate or ethylhexyl acrylate.

Monomers (c) which may be used are those which have two or morepolymerizable double bonds, eg. allyl methacrylate, butanedioldiacrylate, divinylbenzene, triallyl cyanurate anddihydrodicyclopentadienyl acrylate, allyl methacrylate being preferred.

Further examples of monomers (c) are styrene, acrylonitrile, acrylicacid, methacrylic acid, derivatives of the last two, such as methylmethacrylate, acrylamide, methacrylamide, glycidyl acrylate and glycidylmethacrylate, maleic anhydride, maleimide and ethylene. Styrene of theformula ##STR3## where R⁷ and R⁸, which may be identical or different,are each hydrogen or alkyl of 1 to 8 carbon atoms and

n is from 0 to 4, acrylonitrile and/or methyl methacrylate arepreferred.

Further examples of monomers (c) are derivatives of acrylamide and ofmethacrylamide, such as methylolacrylamide methyl ether,N-methylolacrylamide, methylolmethacrylamide methyl ether,N-methylolmethacrylamide, N-methylolmethacrylamide butyl ether andN-methylolmethacrylamide acetate. The mixture (A) contains up to 60,preferably up to 40, % by weight of these monomers.

Examples of the monomers (d) are those already stated for (c),preferably styrene and acrylonitrile. Accordingly, in particular ASAmolding materials are prepared by the novel process.

In the third stage of the novel process, the polymerization can eitherbe carried out continuously as a solution or mass polymerization or canbe continued as a suspension polymerization after a conversion of atleast 15%.

The content of acrylate monomer units in the rubber is at least 30,preferably at least 60, particularly preferably at least 90, % byweight.

In the molding material, the rubbers in grafted form occur as particleshaving a diameter of from 0.1 to 20 μm, preferably from 0.1 to 10 μm, inthe polymer matrix. Bimodal or trimodal distributions are preferred. Therubber particles are in the form of capsules, rods, drops, labyrinths,cells, coils, shells, rod clusters or drop clusters. However, particleswhich consist of a plurality of spherical particles are also observed.Cells and the last-mentioned type are preferred. The stated particleforms are described in A. Echte, Advances in Chemical Serials, 222(1989), 29.

The invention is described in detail below with reference toparticularly preferred embodiments.

EXAMPLES

The following compounds are used in the Examples: cyclohexane, n-butylacrylate, allyl acrylate, acrylonitrile and styrene are products fromBASF and are used without further purification.

Luviskol® K 90 is also a product from BASF and is a polyvinylpyrrolidonehaving a K value of 90, measured in 1% strength solution in water at 25°C. The measurement of the K value is described in Cellulose Chemie, 13(1932), 358-364.

Tetrasodium diphosphate was obtained from Merck, azobisisobutyronitrilefrom Akzo Chemicals and allyl methacrylate from Fluka.

The stabilizer used was octadecyl3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate, a phenolic antioxidantwhich is sold under the name Irganox 1076 by Ciba Geigy.

Ertivinol® is a polyvinyl alcohol from Ercros.

Peroxides

Peroxide 1: tert-Butyl peroxycrotonate

The temperature for a half-life of one hour is 110° C.

Peroxide 2: tert-Butyl 3-isopropenylcumyl peroxide

The temperature for a half-life of one hour is 139° C.

Experiment 1 (according to the invention)

a) Rubber preparation

1100 g of cyclohexane were introduced into a flask and heated to 75° C.under nitrogen and with stirring, and 30 ml of feed 1 and 4 ml of feed 2were then introduced. After 15 minutes, the remainder of the two feedswas added in the course of about 4 hours.

Feed 1

500 g of n-butyl acrylate

4.75 g of allyl methacrylate

8.75 g of peroxide 2 (1.75% by weight, based on rubber)

Feed 2

42 ml of acetone

42 ml of toluene

550 mg of 2,2'-azobisisobutyronitrile

b) Preparation of the end product

The cyclohexane was removed in a rotary evaporator under reducedpressure and replaced with styrene, and acrylonitrile was then added togive a mixture of 69.2% by weight of styrene, 23% by weight ofacrylonitrile and 7.8% by weight of rubber. 1923 g of this solution,together with 2.31 g of Irganox 1076 and 1.35 g of tert-dodecylmercaptan, were introduced into a 5 1 steel kettle and heated to 110° C.under nitrogen and with stirring. After a polymerization time of 105minutes (35-40% conversion), 1.7 g of dicumyl peroxide, 1900 g of water,20 g of Luviskol K 90, 2.0 g of tetrasodium diphosphate and 59.8 g of a10% strength solution of Ertivinol in water were added. Polymerizationof the batch was completed as follows:

at 110° C. for 3 h

at 130° C. for 3 h

at 140° C. for 6 h.

The mixture was then cooled and the polymer was filtered off and dried.

Experiment 2

Experiment 1 was repeated, but with 20 g of peroxide 1 (4% by weight,based on the rubber) instead of 8.75 g of peroxide 2. Thestyrene/acrylonitrile polymerization was carried out at 86° C.

Experiment 3

Experiment 1 was repeated, but 10 g (instead of 8.75 g) of peroxide 2(2% by weight, based on the polymer) were used.

Experiment 4

Comparative Experiment (without peroxides incorporated in the rubber)

The rubber was prepared without peroxides 1 and 2. Dibenzoyl peroxidewas used as a free radical initiator for the polymerization.

The polymerization temperature before the addition of water was 86° C.Otherwise, the procedure was as in Experiment 1.

Testing of the Products

The products were injection molded at a melt temperature of 240° C. anda mold temperature of 60° C. to give standard small bars.

The hole impact strength according to DIN 53753-L-3,0, 4/81 edition, theimpact strength according to DIN 53453-n, 5/75 edition, and the notchedimpact strength according to DIN 53453-k, 5/75 edition, were determinedfor these moldings.

Results

    ______________________________________           % by           weight of                    Impact  Hole im-                                   Notched           peroxides                    strength                            pact   im-pact           1 and 2  at      strength                                   strength                                           Particle    Experi-           in       230° C.                            at 230° C.                                   at 230° C.                                           size    ment   the rubber                    kJ/m.sup.2                            kJ/m.sup.2                                   kJ/m.sup.2                                           distribution    ______________________________________    1.     1.75     32      9.8    2.4     Trimodal:           peroxide                        2 μm;           2                               1 μm;                                           1 μm    2.     4.00     32      10.6   2.6     Trimodal:           peroxide                        5 μm; 1           1                               μm;                                           0.1 μm    3.     2.00     28      9.1    2.4     Bimodal:           peroxide                        1 μm;           2                               0.2 μm    4.     none     16      6.1    1.7    ______________________________________

As can be seen, the peroxides incorporated as polymerized units lead tobetter mechanical properties of the molding materials prepared.

We claim:
 1. A process for the preparation of a rubber-modified moldingmaterial, which comprisesi) in a first stage, polymerizing a firstmixture (A) comprising at least one alkyl acrylate or methacrylate (a)of the formula (I) ##STR4## wherein R¹ is hydrogen or methyl and R² isalkyl of 1 to 32 carbon atoms,at least one first monomer (b) whichcontains a diazo group or a C--C double bond which is sensitive tothermal decomposition, or which monomer (b) is tert.-butyl3-isopropenylcumyl peroxide or tert.-butyl peroxycrotonate, andoptionally a second monomer (c) or a plurality thereof to give a rubber(B), ii) in a second stage, mixing the rubber (B) with a third monomer(d) or a plurality thereof, to form a second mixture (C), and iii) in athird stage, polymerizing the second mixture (C) with free radicals togive the rubber-modified molding material (D), the polymerization beingcarried out either continuously as a solution or mass polymerization orbeing continued as a suspension polymerization after conversion of atleast 15%, the rubber-modified molding material (D) containing from 1 to60% by weight of the rubber (B), and the rubber (B) in grafted formbeing present in the rubber-modified molding material (D) as particleshaving a diameter of from 0.1 to 20 μm.
 2. The process defined in claim1, wherein the mixture (A) is polymerized with free radicals.
 3. Theprocess defined in claim 1, wherein the rubber (B) is dissolved orswollen in the third monomer (d) or a plurality thereof.
 4. The processdefined in claim 1, wherein R² is alkyl of 1 to 18 carbon atoms.
 5. Theprocess defined in claim 1, wherein the mixture (A) is composed offrom30 to 99.95% by weight of the acrylate or methacrylate (a), from 0.05 to10% by weight of the first monomer (b), and from 0 to 60% by weight ofthe second monomer (c).
 6. The process defined in claim 1, for thepreparation of rubber-modified molding material (D) containing from 5 to40% by weight, based on the total amount of (D), of the rubber (B). 7.The process defined in claim 1, wherein the alkyl acrylate ormethacrylate (a) is n-butyl acrylate or ethylhexyl acrylate.
 8. Theprocess defined in claim 1, wherein the second monomer (c) contains twoor more polymerizable double bonds.
 9. The process defined in claim 1,wherein the third monomer (d) is styrene, methyl methacrylate oracrylonitrile or a mixture thereof.
 10. The process defined in claim 1,wherein the second monomer (c) is allyl methacrylate.
 11. The processdefined in claim 10, wherein the second monomer (c) further comprisesstyrene, acrylonitrile or methyl methacrylate.
 12. A rubber-modifiedmolding material (D) which is prepared according to the process definedin claim 1, and wherein the rubber (B) particles form domains.
 13. Therubber-modified material (D) defined in claim 12, wherein the rubber (B)particles form capsules, rods, drops, labyrinths, cells, coils, shells,rod clusters or drop clusters.
 14. A rubber-modified molding material(D) which is prepared according to the process defined in claim 5.